1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, more particularly a positively-charged type photoreceptor for use in electrophotography.
2. Description of the Prior Art
For use as electrophotographic photoreceptors, selenium photoreceptor, or selenium photoreceptor doped with As, Te, Sb or the like, and photoreceptors with zinc oxide or cadmium sulfide dispersed in binder resin are known. These photoreceptors, however, encounter problems in respect of environmental pollution, thermal instability and insufficient mechanical strength.
In recent years, an amorphous silicon (hereinafter referred to as a-Si) based electrophotographic photoreceptor has been proposed. The a-Si has so-called dangling bonds where the Si-Si bond is broken. This type of defect generates many localized energy levels in the energy gap. Because of this, hopping conduction of thermally excited carriers occurs to lower the dark resistance while photoexcited carriers are trapped by the localized energy levels, resulting in poor photoconductivity. It is known to neutralize these defects with hydrogen atoms, namely, to saturate the dangling bonds of silicon atoms with hydrogen atoms.
The hydrogenated amorphous silicon (hereinafter referred to as a-Si:H) exhibits a resistivity of 10.sup.8 to 10.sup.9 .OMEGA.-cm in the dark, which is only about ten thousandth as low as that of amorphous selenium. The photoreceptor comprising a single layer of a-Si:H, therefore, has problems in that its surface potential decays in the dark at a high rate and its initial charge potential is low. a-Si:H, however, has a very favorable characteristic for the photosensitive layer of the photoreceptor in that its resistivity decreases greatly when it is exposed to light in the visible and infrared spectral portions.
To endow such a-Si:H with potential retention, it can be doped with boron to increase its resistivity to a level as high as 10.sup.12 .OMEGA.-cm. However, it is not easy to properly control the boron doping level and others with accuracy. Further, resistivity as high as 10.sup.13 .OMEGA.-cm can be attained by introducing a trace of oxygen with boron. In this case, the photoreceptor exhibits inferior photosensitivity, causing problems such as potential drop in the light and nonnegligible residual potential.
The photoreceptor with a-Si:H exposed in the surface has not yet been fully studied in relation to the chemical stability of its surface, such as, possible influences, as from long-term exposure to atmosphere, or of moisture and chemicals generated under corona discharge. For example, it is known that after having been left to stand for more than a month, it is affected greatly by moisture with a large decrease in the charge potential.
On the other hand, a hydrogenerated amorphous silicon carbide (hereinafter referred to as a-SiC:H) and a method for the production thereof are described in Phil. Mag., Vol. 35 (1978), etc. The material is known to have characteristics such as greater heat resistance, higher surface hardness, and better good dark resistivity (10.sup.12 to 10.sup.13 .OMEGA.-cm) than that of a --Si:H, and an optical energy gap varying between 1.6 and 2.8 eV depending on the carbon content. It however has the disadvantage of inferior sensitivity in the long wavelength region resulting from the windening of the band gap caused by carbon contained therein.
An electrophotographic photoreceptor, as referred to hereinbefore, comprising in combination a-SiC:H and a-Si:H was disclosed, for example, in Japanese laid-open Patent Application No. 127083/1980. It is of two-layer structure, each layer performing a different function, consisting of a photosensitive or photoconductive layer of a-Si:H and a charge transport layer of a-SiC:H under the former. Thus the upper layer plays the role of achieving photosensitivity to light in a wide wavelength region and the lower layer, which is combined heterogenously with the a-Si:H layer, although adapted to improve the charge potential cannot successfully prevent dark decay inherent in a-Si:H layer, with consequent poor charge potential inadequate for practical use. The chemical stability, mechanical strength, heat-proofness, etc. are inferior as attributable to the upper layer of a-Si:H. Besides the charge transport layer remained to be studied relative to the carbon atom content and consideration is given to the thickness of the layers. Accordingly, the electrophotographic receptor does not meet requirements for the desired characteristics. Japanese laid-open Patent Application No. 17952/1982 discloses a three-layer structure photoreceptor, each layer performing a different function, consisting of a photosensitive layer of a-Si:H, a surface modifying layer or the first a-SiC:H layer over the photosensitive layer and a charge transport layer or a second a-SiC:H layer on the opposite side or on the side toward the substrate electrode of the photosensitive layer. It has an advantage of preventing the dark decay inherent in a-Si:H layer. Effects of carbon atom content in the a-SiC:H layer, particularly the charge transport layer, remained to be studied. Thus in either combination with a-Si:H layer, reduced sensitivity and raised residual potential appears. It has too poor stability to produce a large number of copies.